MIMO is a multi-antenna technique that uses multiple antennas at the transmitter and receiver to perform spatial multiplexing. Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a MIMO communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
The LTE standard is currently evolving with enhanced MIMO support. A core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. Currently LTE-Advanced supports an 8-layer spatial multiplexing mode for 8 transmission antennas with channel-dependent precoding. The spatial multiplexing mode is aimed at high data rates in favorable channel conditions.
FIG. 1 is an illustration of the spatial multiplexing operation of a MIMO-OFDM transmitter TX 10 with NT antenna ports 12. An input data stream is separated by a serial-to-parallel converter S/P 14 into its r transmission layers 16. The transmission layer data streams are then coded in a coding unit 18 by multiplying an information carrying symbol vector s by an NT×r precoder matrix W, which serves to distribute the transmit energy into a subspace of the NT-dimensional vector space corresponding to the NT antenna ports. Each transmission layer is precoded with precoder matrix W and then each data stream is passed through an OFDM modulator 20 which performs an IFFT on the r-length blocks. The signals are then sent to each of the NT antennas and transmitted. The r symbols in symbol vector s each correspond to a layer and r is referred to as the transmission rank. In this way, spatial multiplexing is achieved with multiple symbols being transmitted simultaneously over the same time/frequency resource element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
It is conventional that the precoder matrix W is selected from a codebook of possible precoder matrices, and typically indicated by means of a PMI, which specifies a unique precoder matrix in the codebook for a given number of symbol streams. Restriction of the precoder to selection from a codebook is a practical measure to make the amount of computation practical. An ab initio computation of a suitable precoder matrix would take too much processing power.
LTE uses OFDM in the downlink (and DFT precoded OFDM in the uplink). The received NR×1 vector yn for a certain TFRE on subcarrier n (or alternatively data TFRE number n) is modeled by:yn=HnWsn+en  (1)where en is a noise/interference vector obtained as realizations of a random process. The precoder W can be a wideband precoder, which is constant over frequency, or a frequency selective precoder.
The precoder matrix W is often chosen to match the characteristics of the NR×NT MIMO channel matrix Hn, resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially aims to focus the transmit energy into a subspace which is strong in the sense of conveying a large proportion of the transmitted energy to the UE. In addition, the precoder matrix may also be selected with the aim of orthogonalizing the channel, meaning that after proper linear equalization at the UE, the inter-layer interference is reduced.
One example method for a UE to select a precoder matrix W can be to select the Wk that maximizes the Frobenius norm of the hypothesized equivalent channel:
                              max          k                ⁢                                                                                          H                  ^                                n                            ⁢                              W                k                                                          F          2                                    (        2        )            where                Ĥn is a channel estimate, possibly derived from CSI-RS as described below.        Wk is a hypothesized precoder matrix with index k.        ĤnWk is the hypothesized equivalent channel.        
In closed-loop precoding for the LTE downlink, the UE transmits, based on channel measurements in the forward link (i.e. DL), recommendations to the eNodeB of a suitable precoder to use. The eNodeB configures the UE to provide feedback according to the UEs transmission mode, and may transmit CSI-RS and configure the UE to use measurements of CSI-RS to feed back recommended precoding matrices that the UE selects from a codebook. A single precoder that is designed to cover a large band-width (wideband precoding) may be fed back. It may also be beneficial to match the frequency variations of the channel and instead feed back a frequency-selective precoding report, e.g. a report recommending several different precoders, one per subband. This is an example of the more general case of CSI feedback, which also encompasses feeding back other information in addition to precoder recommendations in order to assist the eNodeB in subsequent transmissions to the UE. Such other information may include channel quality indicators (CQIs) as well as a transmission rank indicator (RI).
Based on the CSI feedback from the UE, the eNodeB determines the transmission parameters it wishes to use for transmission to the UE, including the precoding matrix, transmission rank, and modulation and coding state (MCS). Although the transmission parameters determined by the eNodeB take account of the recommendations made by the UE, they may differ from the recommendations taking account of other factors. Therefore, a rank indicator and MCS may be signaled in downlink control information (DCI), and the precoding matrix can be signaled in DCI or the eNodeB can transmit a de-modulation reference signal from which the equivalent channel can be measured. The transmission rank, and thus the number of spatially multiplexed layers, is reflected in the number of columns of the precoder W. For efficient performance, it is important to select a transmission rank which matches the channel properties.
A common type of precoding is to use a DFT-precoder, where the precoder vector used to precode a single-layer transmission using a single-polarized uniform linear array (ULA) with N antennas is defined as
                    w                  1          ⁢          D                    ⁡              (        k        )              =                  1                  N                    ⁡              [                                                            e                                  j                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                                      π                    ·                    0                    ·                                          k                      QN                                                                                                                                              e                                  j                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                                      π                    ·                    1                    ·                                          k                      QN                                                                                                                              ⋮                                                                          e                                  j                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                                      π                    ·                                          (                                              N                        -                        1                                            )                                        ·                                          k                      QN                                                                                                          ]              ,where k=0, 1, (QN−1) is the precoder index and Q is an integer oversampling factor. A corresponding precoder vector for a two-dimensional uniform planar array (UPA) can be created by taking the Kronecker product of two precoder vectors asw2D(k,l)=w1D(k)⊗w1D(l).
Extending the precoder for a dual-polarized UPA may then be done as
            w                        2          ⁢                                          ⁢          D                ,        DP              ⁡          (              k        ,        l        ,                                  ⁢        ϕ            )        =            [                                    1                                                              e                              j                ⁢                                                                  ⁢                ϕ                                                        ]        ⊗                                               w                          2              ⁢                                                          ⁢              D                                ⁡                      (                          k              ,              l                        )                          =                              [                                                  ⁢                                                                                                      w                                              2                        ⁢                                                                                                  ⁢                        D                                                              ⁡                                          (                                              k                        ,                        l                                            )                                                                                                                                                              e                                              j                        ⁢                                                                                                  ⁢                        ϕ                                                              ⁢                                                                  w                                                  2                          ⁢                                                                                                          ⁢                          D                                                                    ⁡                                              (                                                  k                          ,                          l                                                )                                                                                                                  ]                    =                                    [                                                                                                                  w                                                  2                          ⁢                                                                                                          ⁢                          D                                                                    ⁡                                              (                                                  k                          ,                          l                                                )                                                                                                  0                                                                                        0                                                                                                      w                                                  2                          ⁢                                                                                                          ⁢                          D                                                                    ⁡                                              (                                                  k                          ,                          l                                                )                                                                                                        ]                        ⁡                          [                                                                    1                                                                                                              e                                              j                        ⁢                                                                                                  ⁢                        ϕ                                                                                                        ]                                          where ejϕ is a co-phasing factor that may for instance be selected from the QPSK alphabet:
  ϕ  ∈      {          0      ,              π        2            ,      π      ,                        3          ⁢                                          ⁢          π                2              }  
A precoder matrix W2D,DP for multi-layer transmission may be created by appending columns of DFT precoder vectors asW2D,DP=[w2D,DP(k1,l1,ϕ1)w2D,DP(k2,l2,ϕ2) . . . w2D,DP(kR,lR,ϕR)]where R is the number of transmission layers, i.e. the transmission rank. In a common special case for a rank-2 DFT precoder, k1=k2=k and l1=l2=l, meaning that
      W                  2        ⁢                                  ⁢        D            ,      DP        =            [                                                                  w                                                      2                    ⁢                                                                                  ⁢                    D                                    ,                  DP                                            ⁡                              (                                  k                  ,                  l                  ,                                                                          ⁢                                      ϕ                    1                                                  )                                                                                        w                                                      2                    ⁢                                                                                  ⁢                    D                                    ,                  DP                                            ⁡                              (                                  k                  ,                  l                  ,                                                                          ⁢                                      ϕ                    2                                                  )                                                        ]        =                                               [                                                  ⁢                                                                                                      w                                              2                        ⁢                                                                                                  ⁢                        D                                                              ⁡                                          (                                              k                        ,                        l                                            )                                                                                        0                                                                              0                                                                                            w                                              2                        ⁢                                                                                                  ⁢                        D                                                              ⁡                                          (                                              k                        ,                        l                                            )                                                                                            ]                    ⁡                      [                                                            1                                                  1                                                                                                  e                                          j                      ⁢                                                                                          ⁢                                              ϕ                        1                                                                                                                                  e                                          j                      ⁢                                                                                          ⁢                                              ϕ                        2                                                                                                                  ]                          .            
With multi-user MIMO, two or more users in the same cell are co-scheduled on the same time-frequency resource. That is, two or more independent data streams are transmitted to different UEs at the same time, and the spatial domain is used to separate the respective streams. By transmitting several streams simultaneously, the capacity of the system can be increased. This however, comes at the cost of reducing the signal-to-interference-plus-noise power ratio (SINR) per stream, as the power has to be shared between streams and the streams will to some extent interfere with each other.
When increasing the antenna array size, the increased beamforming gain will lead to higher SINR, however, as the user throughput depends only logarithmically on the SINR (for large SINRs), it is instead beneficial to trade the gains in SINR for a multiplexing gain, which increases linearly with the number of multiplexed users.
Accurate CSI is required in order to perform appropriate null-forming between co-scheduled users. In the current LTE Release 13 standard, no special CSI feedback mode for MU-MIMO exists and thus, feedback-based MU-MIMO scheduling and precoder construction has to be based on the existing CSI reporting designed for single-user MIMO (i.e. a PMI indicating a DFT-based precoder, a transmission RI and a CQI). Thus, pairing of users for MU-MIMO and the corresponding link adaptation has to be based on the reported PMI and may for example be derived by calculating the orthogonality of the reported precoders for each user.
The present disclosure is specific to the situation in which an antenna array is subdivided into a plurality of subarrays, wherein each subarray is supplied with a common signal. That is, instead of feeding separate signals to each physical antenna subelement, a plurality of subelements are virtualized together into a subarray, so as to form a virtual antenna element. Each such virtual antenna element, i.e. subarray, is then fed a separate signal.
Dividing an antenna array into subarrays is beneficial so as to reduce the number transmit receive units (TXRUs), which in turn reduces cost. Another purpose is to beamform the antenna ports over which reference signals that define a cell are transmitted; this provides some control of the intercell interference in the network. A yet further example that is frequently used in cellular networks is to use electrical downtilt in the subarrays in order to reduce the interference to adjacent sites.
FIG. 2A shows a standard antenna array 15 which is the aggregate of the individual antennas 12, wherein each antenna is supplied with its own independent signal.
FIG. 2B shows an antenna array 15 which is split into subarrays 25, namely four subarrays consisting of two dual-polarized antennas (or four antenna subelements, two per polarization) each. Each subarray 25 is supplied with a common signal per polarization, i.e. the antennas with the same polarization in any given subarray all receive the same signal. The signal fed to each polarization of a subarray is then mapped onto each constituent antenna subelement with the same polarization of the subarray by some linear function, for instance by applying different phase shifts of the signal to each subelement. For brevity, in the rest of this disclosure, a subarray may refer to one polarization of a subarray, that is fed a single signal.
FIG. 3 shows an example of signal mapping to a subarray of pairs of antennas in which, for each polarization, a phase shift of a is applied to the lower antenna subelement of each antenna pair. This mapping may be described by a matrix multiplication by a Nant×Nvirt matrix Gvirt. In the illustrated example
      G    virt    =            I      8        ⊗          [                                    1                                                              e                                                -                  j                                ⁢                                                                  ⁢                α                                                        ]      where I8 is a size 8×8 identity matrix, so that x=Gvirty where x and y are the signals mapped to the antenna subelements and subarrays, respectively.
The antenna ports described herein are assumed to be defined by reference signals transmitted on the subarrays.
When subarrays are employed, existing methods for pairing of users for MU-MIMO have the short-coming that they rely on determining the orthogonality of the co-scheduling candidates among reported precoders. This may lead to suboptimal link adaptation and to suboptimal user pairing when subarrays are used, as neither the subarray radiation pattern, nor the presence of grating lobes are taken into account. Grating lobes arise if the condition d≥λ/2 is met, where λ is the carrier wavelength, which is often the case when subarrays are provided. Grating lobes may be considered to be analogous to aliasing as a result of undersampling and mean that there are multiple candidate directions for the transmission direction (or bearing), only one of which is correct.